Why does Grease Pencil ignore the particle system in Eevee? (2.82)
I found the possible culprit of the problem and the solution:

In the material properties-tab of the material (in this case the leaves) I found the solution.

 

Because I used a png-file of the leaves with transparency I opted for the Alpha Blend. But somehow the 'backface Culling' was ticked.
This was NOT a good idea. When unchecking the box it worked.



Although I got better results with the hashed option in the Blend mode.